This invention relates to a piston particularly utilized in an internal combustion engine and more particularly to an improved alloy for such pistons and method for manufacturing them.
As is well known, the piston for an internal combustion engine has a number of rigid and yet diverse requirements. That is, the piston should be light in weight and yet also high in strength. The sliding surfaces of the piston should provide low friction and be able to withstand high compressive forces. Furthermore, different areas of the piston have quite different conditions which they must withstand. For example, the head of the piston must be capable of withstanding high temperatures as occur during combustion and also the coefficient of thermal expansion should be controlled so as to minimize the differences in dimensional clearances between the piston and the cylinder bore during temperature changes as occur in engine operation.
Conventionally, aluminum has been utilized as the basic material for pistons in engines. In order to improve the characteristics of the piston, frequently silicon (Si) is employed as an alloying element. By adding silicon, the ability to cast the piston can be improved since the melting point is lowered and the flow of molten material is facilitated. Also, the silicon resists deformation at high temperatures by lowering the coefficient of thermal expansion. In addition, resistance against wear and fatigue under high speed sliding action is improved.
Certain of these characteristics such as the lowering of the thermal expansion and the improved resistance against wear are somewhat proportional to the amount of silicon used in the alloy. Thus, the output of the engine and thermal load on the piston the greater amount of silicon is added.
However, silicon has a considerably lower thermal conductivity than aluminum. Thus, aluminum alloys having large amounts of silicon have low thermal conductivity. Therefore, heat dissipation is deteriorated and thus, overheating particularly in the head area results. In fact, the degree of overheating may be such that actual melting of the piston head may occur.
It is, therefore, a principal object of this invention to provide an aluminum alloy for use in forming pistons that will provide improved strength and heat dissipation performance.
It is a further object of this invention to provide an aluminum alloy for use in forming pistons that will increase thermal conductivity without increasing the coefficient of thermal expansion and to improve resistance against wear and fatigue strength, particularly under conditions of high temperature and high speed.
In conjunction with the formation of the piston from the material, it is also important to ensure that the properties of the material and way in which the piston is formed is such that cracks cannot develop during the formation process. The use of silicon and silicon and certain other alloying materials can give rise to problems in connection with forming the piston which can result in defects being created in the actual forming process.
It is, therefore, a still further object of this invention to provide an improved method for forming a piston.